1. Field of the Invention
The present invention relates to projection exposure method and apparatus used in forming fine patterns for semiconductor integrated circuits, liquid crystal displays, or the like.
2. Related Background Art
A projection exposure apparatus as called as a stepper is provided with a projection optical system. This projection optical system is incorporated into the exposure apparatus after it is produced in the steps of high-level optical design, strict selection of glass materials, superfine machining of glass materials, and precise assembly and adjustment.
Current steppers actually used in fabrication of semiconductors are so arranged that illumination light of i-line (wavelength 365 nm) from a mercury lamp illuminates a photomask as called as a reticle and that a projection optical system focuses light passing through a circuit pattern on the reticle to form an image thereof on a photosensitive substrate such as a wafer. On the other hand, excimer steppers employing the excimer laser (KrF laser of wavelength 248 nm) are used for evaluation or for research. If a projection optical system for excimer stepper is composed only of refracting lenses, the glass materials usable therefor are limited to quartz or fluorite.
Generally, the resolving power and the depth of focus (D.O.F.) of the projection optical system are important factors to faithfully transfer a fine reticle pattern onto a photosensitive substrate upon exposure with the projection optical system. Some of presently available projection optical systems for i-line have a numerical aperture (N.A.) of about 0.6. If the wavelength of used illumination light is kept constant, the resolving power increases as the numerical aperture of projection optical system increases. The depth of focus (D.O.F.), however, decreases as the numerical aperture increases. The depth of focus is defined by D.O.F.=.+-..lambda./(N.A.).sup.2 where .lambda. is the wavelength of illumination light.
FIG. 1 schematically shows optical paths of image formation in a conventional projection optical system. The projection optical system is composed of a front-group lens system GA and a rear-group lens system GB. The projection optical system of this type is generally both-side telecentric on the reticle R side and on the wafer W side or single-side telecentric only on the wafer side.
There are rays L1, L2, L3, La, La', La" advancing in various directions from a point A on the pattern surface of reticle (i.e., the object plane of projection optical system), among which the ray L1 is traveling at an angle at which it cannot enter the front-group lens system GA. Also, among the rays entering the front lens system GA, the rays L2, L3 cannot pass through a pupil ep (circular region about the optical axis AX) located on the Fourier transform plane FTP in the projection optical system. The other rays La, La', La" pass through the pupil ep and then enter the rear-group lens system GB to converge at a point A' on the surface of wafer. Accordingly, a point image is formed at the point A' by only the rays passing through the pupil ep in the projection optical system out of the rays leaving the point A on the reticle.
Out of the rays traveling from the point A to the point A', the ray La passing through the center point CC of pupil is called as a principal ray. If the projection optical system is telecentric on the both sides, the principal ray La travels in parallel with the optical axis AX in the both spaces on the object plane side and on the image plane side.
The above is also the case with rays leaving another point B or C on the reticle, and a point image is thus formed at point B' or C' only by rays passing through the pupil similarly, a ray Lb, Lc travels in parallel with the optical axis AX from the point B, C to enter the lens system GA, becoming a principal ray passing through the center point CC of pupil.
As described, the pupil is in relation of Fourier transform or inverse transform with the pattern surface of reticle or with the surface of wafer so that among the rays from the pattern on the reticle the rays contributing to image formation all superimposedly pass through the pupil.
Discussing the numerical aperture of projection optical system, there are a numerical aperture NAw on the wafer side (on the image plane side) and a numerical aperture NAr on the reticle side (on the object side). The numerical aperture NAw on the wafer side corresponds to an angle .theta..sub.w which the ray La' or La" passing through the outermost portion of pupil among the image-forming rays toward the point A' makes with the principal ray La on the wafer. The numerical aperture NAw is expressed as NAw=sin.theta.w. Also, the numerical aperture NAr on the reticle side corresponds to an angle .theta.r which the ray La' or La" makes with the principal ray La on the reticle side. The numerical aperture NAr is expressed as NAr=sin.theta.r. Further, there is a relation of NAr=M.multidot.NAw where M is an imaging magnification of projection optical system.
Incidentally, the resolving power can be enhanced by increasing the numerical aperture NAw or NAr. In other words, the resolving power can be increased by making the diameter of pupil larger and further making the effective diameter of lens system GA, GB larger. Nevertheless, the depth of focus (D.O.F.) decreases in proportion to an inverse of square of numerical aperture. Then, even if the projection optical system could be produced with high numerical aperture, the depth of focus would be insufficient, which would be a hindrance in practical use.
When the illumination light is the i-line of wavelength 365 nm and the numerical aperture NAw is 0.6, D.O.F. is about 1 .mu.m (.+-.0.5 .mu.m) across. As a result, the resolution is poor in a portion where the surface unevenness or curvature exceeds the D.O.F. in a shot area (a square of about 20 mm to 30 mm) on the wafer. As for the system of stepper, the focusing and leveling must be carried out with considerably high precision for each shot area on the wafer, which increases loads on the mechanical system, the electric system and the software (that is, requiring efforts to improve the measurement resolution, the servo control precision, the setting time, and so on).
Applicant proposed a novel projection exposure technology which can provide both a high resolving power and a deep depth of focus, solving such various problems in the projection optical system, as described in Japanese Laid-open Patent Application No. 4-101148 (corresponding to a United States Patent Application, U.S. Ser. No. 847,030 filed on Apr. 15, 1992) or in Japanese Laid-open Patent Application No. 4-225358 (corresponding to a United States Patent Application, U.S. Ser. Number 791,138 filed on Nov. 13, 1991) for example. This exposure technology increases an apparent resolving power and an apparent depth of focus by controlling illumination in a special shape on the reticle without changing the projection optical system, which is called as SHRINC (Super High Resolution by Illumination Control) method.
In the SHRINC method the reticle is illuminated by two or four illumination beams symmetrically inclined with respect to the pitch direction of line-and-space pattern on the reticle. The zeroth-order-diffracted-light component and one of .+-.first-order-diffracted-light components, which are emergent from the line-and-space pattern, are let to pass through the pupil ep of projection optical system symmetrically with each other with respect to the center point CC therein. Interference (double beam interference) effect occurs between the zeroth-order-diffracted light and one of the first-order-diffracted light to produce a projection image (i.e., interference fringes) of the line-and-space pattern. Such image formation utilizing the double beam interference can suppress occurrence of wavefront aberration in a defocus state, more than in ordinary perpendicular illumination in conventional systems, increasing the apparent depth of focus.
The SHRINC method can enjoy the desired effect with a periodic pattern formed on the reticle, but no such effect can be expected for an isolated pattern such as a contact hole. Generally, in case of an isolated fine pattern, most of diffracted light therefrom is due to the Fraunhofer's diffraction, so that the diffracted light is not clearly split into the zeroth-order-diffracted light and higher-order-diffracted light in the pupil ep of projection optical system.
Then there is an exposure method for increasing the apparent depth of focus for isolated pattern such as contact hole, proposed for example in U.S. Pat. No. 4,869,999, in which exposure is effected at a plurality of steps for each shot area on the wafer and the wafer is moved by a constant amount along the optical axis between steps. This exposure method is called as FLEX (Focus Latitude Enhancement Exposure) method, which is fully effective to increase the depth of focus for isolated pattern such as contact hole. However, sharpness of resist image obtained after development is inevitably lowered, because the FLEX method requires multiple exposure of contact hole image in a slightly defocused state. This problem of sharpness decrease (profile degradation) can be compensated for by using a resist with high gamma value, a multi layer resist, or a Contrast Enhancement Layer (CEL).
There is an attempt known to increase the depth of focus in projection of contact hole pattern without moving the wafer along the optical axis in exposure process as in the FLEX method, for example the Super-FLEX method as presented in Extended Abstracts 29a-ZC-8, 9 (The Spring Meeting, 1991); The Japan Society of Applied Physics and Related Societies. In the Super-FLEX method, a transparent phase plate is provided in the pupil of projection optical system so that the phase plate gradually changes the complex amplitude transmittance of imaging light from the optical axis toward the periphery thereof. Such arrangement can permit the image focused by the projection optical system to keep sharpness within a certain range along the optical axis about the best focus plane (which is a plane conjugate with the reticle), increasing the depth of focus.
Among the various conventional techniques as described above, the FLEX method and the Super-FLEX method can obtain a sufficient increase in depth of focus for isolated contact hole pattern. It is, however, found that with a plurality of contact hole patterns closely juxtaposed at a certain distance either one of the above methods causes undesirable film reduction in photoresist between the holes and that they cannot be virtually used for close contact hole patterns.
Further, the FLEX method can increase the depth of focus for isolated contact hole pattern on one hand, while it has such a problem that the exposure dose latitude or margin has to be decreased to maintain the sharpness of synthetic optical image obtained by multiple exposure on the other hand. The FLEX method also has other problems such as difficult application to exposure apparatus of scanning exposure method or a great drop in processing capability.